Phillip Bolton, an opthalmic surgeon with the Palmerston North Hospital, responded.

In order to maintain a healthy and optically clear cornea, (the front surface
of our eye) an adequate supply of tears is needed to cope with evaporation.
Tears are constantly and evenly distributed over the cornea by blinking.

The tear film layer consists of an oily outer layer to reduce evaporation and
provide lubrication for eyelid movement, an aqueous component which contains
electrolytes, nutrients and antimicrobial substances and a layer of mucin
which is hydrophilic and helps to maintain an even thin layer of moisture over
the cornea with each blink.

Tears are a fluid secreted by various types of glands of the eyelid margins
and other secreting elements within the conjunctiva, the thin layer lining
the eyelids and the surface of the eyeball of the eye outside the cornea.

The tear secretion system has two components. Firstly, basic secretors with no
nerve supply. These are all the small accessory glands in the conjunctiva and
eye lid margins. Eye movement and blinking causes these glands to emit their
secretions. Secondly, the reflex secretor, the main lacrimal gland, which has a
nerve supply from the autonomic nervous system. Excessive tear flow can be
stimulated by bright light, emotion or a foreign body or irritating substance.
These extra tears may pass down the drainage system into the nose causing a
`running nose' or overflow down the cheeks as in crying.

If inadequate tears are produced naturally, then the supplementary use of
eyedrop artificial tears may be recommended.